When people genetically modify animals, they can mess up a lot of their natural abilities. For example in some cases scientist have made cows who produce more milk, but couldn't reproduce anymore because of it. Sometimes it even hurts the animal. People have bred dogs in certain ways that makes it hard for them to breathe, like Bull dogs who has respiratory issues. This makes decreases their life span and continuing to bred them will only make it worse. When scientist have a certain trait they want they also make animals breed with their own families. Incest creates a lot of genetic problems.
